Added BUILD.gn file for flutter_tools on Fuchsia. (#6014)

packages/flutter_tools/BUILD.gn

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/dart/dart_package.gni")
+
+dart_package("flutter_tools") {
+  package_name = "flutter_tools"
+
+  source_dir = "lib"
+
+  deps = [
+    "//third_party/dart-pkg/pub/analyzer",
+    "//third_party/dart-pkg/pub/archive",
+    "//third_party/dart-pkg/pub/args",
+    "//third_party/dart-pkg/pub/coverage",
+    "//third_party/dart-pkg/pub/crypto",
+    "//third_party/dart-pkg/pub/file",
+    "//third_party/dart-pkg/pub/http",
+    "//third_party/dart-pkg/pub/json_rpc_2",
+    "//third_party/dart-pkg/pub/json_schema",
+    "//third_party/dart-pkg/pub/linter",
+    "//third_party/dart-pkg/pub/meta",
+    "//third_party/dart-pkg/pub/mustache4dart",
+    "//third_party/dart-pkg/pub/package_config",
+    "//third_party/dart-pkg/pub/path",
+    "//third_party/dart-pkg/pub/pub_semver",
+    "//third_party/dart-pkg/pub/stack_trace",
+    "//third_party/dart-pkg/pub/usage",
+    "//third_party/dart-pkg/pub/web_socket_channel",
+    "//third_party/dart-pkg/pub/xml",
+    "//third_party/dart-pkg/pub/yaml",
+    "//third_party/dart-pkg/pub/test",
+  ]
+}